The present invention is related to MOS semiconductor devices and, more particularly, to a power MOS semiconductor device with continuous body contact and a wide active channel.
Co-pending, commonly assigned U.S. application Ser. No. 09/260,411, filed Mar. 1, 1999 by Christopher B. Kocon et al. for MOS-GATED DEVICE HAVING A BURIED GATE AND PROCESS FOR FORMING SAME, the disclosure of which is incorporated herein by reference, describes a device expected to provide high cell packing density through the use of a recessed and buried gate. FIG. 1 (FIG. 2 in application Ser. No. 09/260,411) depicts a trench MOS-gated device 200 that includes a doped N+ substrate 201 on which is deposited an epitaxial doped upper layer 202. Epitaxial layer 202 includes drain region 203, heavily doped P+ body regions 204, and P-well regions 205. Abutting body regions 204 in epitaxial layer 203 are heavily doped N+ source regions 206, which are separated from each other by a gate trench 207 that has dielectric sidewalls 208 and floor 209. Contained within trench 207 is a gate material 210, filled to a selected level 211, and an overlying dielectric layer 212. Selected level 211 of gate material 210 is approximately coplanar with the selected depth 216 of N+ source regions 206, thereby providing overlap between source regions 206 and gate material 210. The surface 213 of gate dielectric layer 212 is substantially coplanar with the surface 214 of epitaxial layer 202. Deposited metal layer 215 contacts body regions 204 and source regions 206.
FIG. 2 (FIG. 3B in application Ser. No. 09/260,411) depicts an alternative prior art trench MOS-gated device 300 that includes a doped N+ substrate 301, on which is disposed a doped upper layer 302. Upper layer 302 includes drain region 303 and P-wells 305. N+ source regions 306, formed by ion implantation and diffusion to a selected depth 316 in upper layer 302, are also separated by gate trench 307. Gate trenches 307 each have dielectric sidewalls 308 and a floor 309 and contain conductive gate material 310, filled to a selected level 311, and an overlying dielectric layer 312. The surface 313 of gate dielectric layer 312 is substantially coplanar with the surface 314 of upper layer 302. Metal layer 315 is deposited on surface 314 to contact body regions 304 and source regions 306.
Although the just-described prior art structures are expected to provide high cell packing density, the periodic placement of the P+ body regions produces a high series resistance, resulting in degradation of the electro-thermal dynamic characteristics and ruggedness as well as SOA (Safe Operation Area) of the devices. Also, depending on the total area of the P+ body regions, some loss of channel width can result.
The present invention is directed to an MOS power device a substrate that comprises an upper layer having an upper surface and an underlying drain region, a well region of a first conductance type disposed in the upper layer over the drain region, and a plurality of spaced apart buried gates, each of which comprises a trench that extends from the upper surface of the upper layer through the well region into the drain region. Each trench comprises an insulating material lining its surface, a conductive material filling its lower portion to a selected level substantially below the upper surface of the upper layer, and an insulating material substantially filling the remainder of the trench. A plurality of highly doped source regions of a second conductance type are disposed in the upper layer adjacent the upper portion of each trench, each source region extending from the upper surface to a depth in the upper layer selected to provide overlap between the source regions and the conductive material in the trenches. A xe2x80x9cVxe2x80x9d groove in each of the highly doped source regions extends through the source regions into the well region and terminates in a nadir. A highly doped body region of a first conductance type is disposed in the well region adjacent both to the nadir of one or more of the grooves and to adjacent source regions penetrated by the grooves. A conductive layer is disposed over the substrate and electrically contacts the body and source regions.
The present invention is further directed to a process for fabricating an MOS power device that comprises: providing a semiconductor substrate comprising an upper layer that has an upper surface and an underlying drain region, and forming a well region of a first conductance type in the upper layer overlying the drain region. A plurality of spaced apart gate trenches, each extending from the upper surface of the upper layer through the well region into the drain region, are formed and lined with an insulating material. A lower portion of each said trench is filled with a conductive material to a selected level substantially below the upper surface of the upper layer, and the upper portion of each trench is substantially filled with an insulating material, thereby forming a plurality of trench gates.
A plurality of highly doped source regions of a second conductance type are formed in the upper layer adjacent the upper portion of each trench, each source region extending from the upper surface to a depth in the upper layer selected to provide overlap between the source regions and the conductive material in the trenches. A xe2x80x9cUxe2x80x9d groove is formed in each of the highly doped source regions, each groove extending through the source region into the well region and terminating in a nadir. A highly doped body region of a first conductance type is implanted in the well region adjacent the nadir of one or more of the grooves, and also adjacent source regions penetrated by the grooves. A conductive layer is deposited over the substrate for electrically contacting the body and source regions.
The MOS power device of the present invention, which is formed by a completely self-aligned process, avoids the loss of channel width and provides reduced channel resistance without sacrificing device ruggedness and dynamic characteristics, as well as SOA.